1. Field of the Invention
The present invention relates to elastic wave apparatuses in which an elastic wave filter element used as a bandpass filter is disposed on a multilayer wiring board, and, more particularly, to an elastic wave apparatus including a balanced-unbalanced transforming function and a duplexer including the elastic wave apparatus.
2. Description of the Related Art
Elastic wave apparatuses such as surface acoustic wave filters have been recently used as, for example, bandpass filters in RF stages of mobile telephones.
Japanese Unexamined Patent Application Publication No. 2002-84165 discloses a surface acoustic wave apparatus as an example of such an elastic wave apparatus.
FIG. 10 is a schematic plan view illustrating an electrode structure of a surface acoustic wave apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-84165. A surface acoustic wave apparatus 1001 is obtained by forming illustrated electrodes on a piezoelectric substrate. The surface acoustic wave apparatus 1001 includes an unbalanced terminal 1002, a first balanced terminal 1003, and a second balanced terminal 1004, and has a balanced-unbalanced transforming function.
In the surface acoustic wave apparatus 1001, a first longitudinally coupled resonator-type surface acoustic wave filter 1005 and a second longitudinally coupled resonator-type surface acoustic wave filter 1006 are cascade-connected. The first surface acoustic wave filter 1005 includes a first IDT 1005a, a second IDT 1005b, and a third IDT 1005c which are disposed along a surface acoustic wave propagation direction. The second surface acoustic wave filter 1006 includes a first IDT 1006a, a second IDT 1006b, and a third IDT 1006c which are disposed along a surface acoustic wave propagation direction. A reflector 1005d is disposed on one side of an area where the first IDT 1005a to the third IDT 1005c are disposed in the surface acoustic wave propagation direction, and a reflector 1005e is disposed on the other side of the area. The second surface acoustic wave filter 1006 also includes reflectors 1006d and 1006e. 
One end of the second IDT 1005b is connected to the unbalanced terminal 1002. One end of the first IDT 1005a is connected to one end of the first IDT 1006a via a first line 1007. One end of the third IDT 1005c is connected to one end of the third IDT 1006c via a capacitor 1008. One end of the second IDT 1006b is connected to the first balanced terminal 1003, and the other end of the second IDT 1006b is connected to the second balanced terminal 1004. That is, a pair of balanced outputs are obtained from the second surface acoustic wave filter 1006 of a floating balanced type.
In the surface acoustic wave apparatus 1001, a capacitor 1008 is connected between the first balanced terminal 1003 and the second balanced terminal 1004. The capacitor 1008 corrects the difference in frequency characteristic between balanced signals obtained from the first balanced terminal 1003 and the second balanced terminal 1004. As a result, an amplitude balance and a phase balance can be improved.
Japanese Unexamined Patent Application Publication No. 2002-84165 also discloses a configuration illustrated in FIG. 11 in addition to a case in which an external capacitor element is used as the capacitor 1008. FIG. 11 is a schematic bottom cross-sectional view of a die attach portion in a package 1009. In the die attach portion in the package 1009, electrodes 1010a to 1010d are formed to establish external electric connection. The electrodes 1010c and 1010d are electrically connected to the first balanced terminal 1003 and the second balanced terminal 1004, respectively. The electrodes 1010c and 1010d are adjacent to each other, so that the capacitor 1008 is formed with a capacitance generated between the electrodes 1010c and 1010d. 
When the surface acoustic wave apparatus 1001 is used as a receiver bandpass filter in a duplexer in a mobile telephone, a transmitter bandpass filter is also disposed in the package 1009. Accordingly, when the size of the package 1009 is reduced, it is difficult to freely dispose the electrodes 1010c and 1010d in the die attach portion. It is therefore difficult to obtain a sufficient capacitance. That is, it is difficult to obtain sufficient capacitance to improve the balance between the electrodes 1010c and 1010d. 
Furthermore, when the size of the package 1009 is reduced, it is difficult to freely dispose electrodes and lines in a piezoelectric substrate and freely determine the positions of the first balanced terminal 1003 and the second balanced terminal 1004. It is therefore difficult to symmetrically dispose balanced signal lines that are individually connected to the first balanced terminal 1003 and the second balanced terminal 1004. This may lead to deterioration in balance.